Electrophoretic fluid

ABSTRACT

The present invention is directed to a display fluid comprising charged composite pigment particles dispersed in a solvent. The composite pigment particles have a density which matches to the density of the solvent in which they are dispersed. A display fluid comprising the composite pigment particles provides improved display performance.

This application is a continuation of U.S. patent application Ser. No.13/549,028, filed Jul. 13, 2012; which is a continuation-in-part of U.S.patent application Ser. No. 13/363,741, filed Feb. 1, 2012; which claimspriority to U.S. Provisional Application No. 61/439,302, filed Feb. 3,2011; the contents of the above applications are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The present invention is directed to the preparation of compositepigment particles that can be used to form an electrophoretic fluid andthe resulting display fluid.

BACKGROUND OF THE INVENTION

An electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon influencing charged pigment particlesdispersed in a dielectric solvent. An EPD typically comprises a pair ofspaced-apart plate-like electrodes. At least one of the electrodeplates, typically on the viewing side, is transparent. Anelectrophoretic fluid composed of a dielectric solvent with chargedpigment particles dispersed therein is enclosed between the twoelectrode plates.

An electrophoretic fluid may have one type of charged pigment particlesdispersed in a solvent or solvent mixture of a contrasting color. Inthis case, when a voltage difference is imposed between the twoelectrode plates, the pigment particles migrate by attraction to theplate of polarity opposite that of the pigment particles. Thus, thecolor showing at the transparent plate may be either the color of thesolvent or the color of the pigment particles. Reversal of platepolarity will cause the particles to migrate back to the opposite plate,thereby reversing the color.

Alternatively, an electrophoretic fluid may have two types of pigmentparticles of contrasting colors and carrying opposite charges, and thetwo types of pigment particles are dispersed in a clear solvent orsolvent mixture. In this case, when a voltage difference is imposedbetween the two electrode plates, the two types of pigment particleswould move to the opposite ends (top or bottom) in a display cell. Thusone of the colors of the two types of the pigment particles would beseen at the viewing side of the display cell.

In another alternative, color pigment particles are added to anelectrophoretic fluid for forming a highlight or multicolor displaydevice.

For all types of the electrophoretic displays, the fluid containedwithin the individual display cells of the display is undoubtedly one ofthe most crucial parts of the device. The composition of the fluiddetermines, to a large extent, the lifetime, contrast ratio, switchingrate and bistability of the device.

In an ideal fluid, the charged pigment particles remain separate and donot agglomerate or stick to each other or to the electrodes, under alloperating conditions. In addition, all components in the fluid must bechemically stable and compatible with the other materials present in anelectrophoretic display.

Currently, the pigment particles in an electrophoretic fluid often havea density which is much higher than that of the solvent in which theparticles are dispersed, thus causing performance issues, such as poorgrey level bistability, vertical driving and settling phenomena.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show the composite pigment particles of the presentinvention.

FIG. 2 shows reaction steps of a process suitable for the preparation ofthe composite pigment particles of the present invention.

SUMMARY OF THE INVENTION

The present invention is directed to a display fluid comprising chargedcomposite pigment particles dispersed in a solvent, wherein each of saidcomposite pigment particles comprises at least a core pigment particle,a shell coated over the core pigment particle and steric stabilizermolecules on the surface of the composite pigment particles.

In one embodiment, the density of the composite pigment particlessubstantially matches to that of the solvent.

In one embodiment, the difference between the density of the compositepigment particles and the density of the solvent is less than 2 g/cm³.

In one embodiment, the core pigment particle is an inorganic pigmentparticle and the core pigment particles may be surface treated orsurface untreated. In this embodiment, the shell of the core pigmentparticle may be formed from an inorganic material, and if so, theorganic content of the composite pigment particles may be in the rangeof about 10% to about 50% by weight, preferably more than about 15% upto about 30% by weight. The shell may also be formed from an organicmaterial, and in that case, the organic content of the composite pigmentparticles may be at least about 20% by weight, preferably about 20% toabout 70% by weight and more preferably about 20% to about 40% byweight.

In one embodiment, the core pigment particle may be an organic pigmentparticle. In this embodiment, the core pigment particle may also besurface treated or surface untreated. The polymer content of thecomposite pigment particles with an organic core particle may be atleast about 20%, preferably about 30% to about 70% by weight and morepreferably about 40% to about 60% by weight.

In one embodiment, the shell may be completely incompatible orrelatively incompatible with the solvent.

In one embodiment, the steric stabilizer molecules may be formed frompolyacrylate, polyethylene, polypropylene, polyester, polysiloxane or amixture thereof.

In one embodiment, the surface of the shell may comprise functionalgroups to enable charge generation or interaction with a charge controlagent.

In one embodiment, the fluid may further comprise a second type ofcharged pigment particles. In one embodiment, the second type of chargedpigment particles is composite pigment particles comprising at least acore pigment particle, a shell coated over the core pigment particle andsteric stabilizer molecules on the surface of the composite pigmentparticles. The two types of composite pigment particles in the fluid areof contrasting colors. In one embodiment, the fluid may comprise morethan two types of pigment particles and each type has a color differentfrom the colors of other types.

In one embodiment, the solvent in which the composite pigment particlesare dispersed may be a hydrocarbon solvent or a mixture of a hydrocarbonsolvent and another solvent, such as a halogenated solvent or a siliconeoil type solvent.

In one embodiment, the composite pigment particles may be prepared bydispersion polymerization or living radical polymerization.

DETAILED DESCRIPTION OF THE INVENTION

The first aspect of the present invention is directed to the compositepigment particles, as shown in FIGS. 1a and 1b . The composite pigmentparticles are closely density matched to a solvent in which they aredispersed, especially in a hydrocarbon solvent.

The composite pigment particles (10) may have one or more core pigmentparticles (11). The core particle(s) (11) is/are coated with a shell(12). There are steric stabilizer molecules (13) on the surface of thecomposite pigment particles. The core pigment particles may be of anycolors (e.g., black, white, red, green, blue, cyan, magenta, yellow orthe like).

The core particles may be formed from an inorganic material, such asTiO₂, BaSO₄, ZnO, metal oxides, manganese ferrite black spinel, copperchromite black spinel, carbon black or zinc sulfide pigment particles.

The inorganic core particles may be optionally surface treated. Thesurface treatment would improve compatibility of the core pigmentparticles to the monomer in a reaction medium or chemical bonding withthe monomer, in forming the composite pigment particles. As an example,the surface treatment may be carried out with an organic silane havingfunctional groups, such as acrylate, vinyl, —NH₂, —NCO, —OH or the like.These functional groups may undergo chemical reaction with the monomers.

The shell may be formed from an inorganic or organic material.

Inorganic shell materials may include silica, aluminum oxide, zinc oxideand the like or a combination thereof. Sodium silicate ortetraethoxysilane may be used as a common precursor for silica coating.

An organic shell may be formed from an organic polymer, such aspolyacrylate, polyurethane, polyurea, polyethylene, polyester,polysiloxane or the like. For example, a polyacrylate shell may beformed from monomer, such as styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylateor the like. A polyurethane shell may be formed from monomer oroligomer, such as multifunctional isocyanate or thioisocyanate, primaryalcohol or the like. A polyurea shell may be formed from monomercontaining reactive groups, such as amine/isocyanate,amine/thioisocyanate or the like. A person skilled in the art would beable to select proper monomer or oligomer and its variations, based onthe main idea of the present invention.

If the shell is inorganic, the structure of the shell may be porous toreduce density. The “organic content” of the resulting composite pigmentparticles from inorganic core particles would be in the range of about10% to about 50% by weight, preferably more than about 15% up to about30% by weight. In this embodiment, the term “organic content” isdetermined by the weight of the steric stabilizers (13) divided by thetotal weight of the core pigment particles (11), the shell (12) and thesteric stabilizers (13).

If the shell is organic, the “organic content” of the resultingcomposite pigment particles from inorganic core particles would be atleast about 20% by weight, preferably about 20% to about 70% by weightand more preferably about 20% to about 40% by weight. In thisembodiment, the term “organic content” is determined by the total weightof the shell (12) and the steric stabilizers (13) divided by the totalweight of the core pigment particles (11), the shell (12) and the stericstabilizers (13).

The density of the resulting shell, in any case, is preferably low,lower than 2 g/cm³ and more preferably about 1 g/cm³. The shellthickness may be controlled, based on the density of the shell materialand the desired final particle density.

The shell material is either completely incompatible or relativelyincompatible with the display fluid in which the composite pigmentparticles are dispersed. Relatively incompatible” as used herein, meansthat no more than about 5%, preferably no more than about 1%, of theshell material is miscible with the display fluid.

In order to achieve this complete or relative incompatibility, the shellpolymer material may have polar functionality on its main chain or aside chain. Examples of such polar functionality may include —COOH, —OH,NH₂, R—O—R, R—NH—R and the like (wherein R is an alkyl or aryl group).Each of the side chains, in this case, preferably has less than 6 carbonatoms. In one embodiment, the main chain or the side chain may containan aromatic moiety.

In addition, the core pigment particle(s) and the shell should behave asone single unit. This may be achieved by cross-linking or anencapsulation technique, as described below.

The steric stabilizer (13) in FIG. 1 is usually formed of high molecularweight polymers, such as polyethylene, polypropylene, polyester,polysiloxane or a mixture thereof. The steric stabilizer facilitates andstabilizes the dispersion of the composite pigment particles in asolvent.

Furthermore, the surface of the shell may optionally have functionalgroups that would enable charge generation or interaction with a chargecontrol agent.

In another embodiment of the present invention, the core particles maybe formed from an organic material, such as CI pigment PR 254, PR122,PR149, PG36, PG58, PG7, PY138, PY150, PY20 or the like, which arecommonly used organic pigment materials described in the color indexhandbook “New Pigment Application Technology” (CMC Publishing Co, Ltd,1986) and “Printing Ink Technology” (CMC Publishing Co, Ltd, 1984).Specific examples may include Clariant Hostaperm Red D3G 70-EDS,Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70, BASFIrgazine red L 3630, Cinquasia Red L 4100 HD, Irgazin Red L 3660 HD andthe like. The composite pigment particles formed from the organic coreparticles are usually colored, such as red, green, blue, cyan, magenta,yellow or the like.

The surface of the organic core particles may be treated or untreated.The surface treatment would improve compatibility of the core pigmentparticles to the monomer in a reaction medium or chemical bonding withthe monomer, in forming the composite color particles. The pre-treatedfunctional molecules can be either chemically bonded or physicallyabsorbed onto pigment surface. The functional molecules may be adispersant, surfactant or the like.

The shell for organic core particles is usually formed from an organicshell material as described above. The stabilizers for the compositepigment particles prepared from organic core particles may also beprepared as described below.

The “polymer content” of the composite pigment particles prepared fromorganic core particles may be at least about 20% by weight, preferablyabout 30% to about 70% by weight and more preferably about 40% to about60% by weight. The term “polymer content” is determined by the totalweight of the shell (12) and the steric stabilizers (13) divided by thetotal weight of the core pigment particles (11), the shell (12) and thesteric stabilizers (13).

The second aspect of the present invention is directed to thepreparation of the composite pigment particles of the present invention,which may involve a variety of techniques.

For example, they may be formed by dispersion polymerization. Duringdispersion polymerization, monomer is polymerized around core pigmentparticles in the presence of a steric stabilizer polymer soluble in thereaction medium. The solvent selected as the reaction medium must be agood solvent for both the monomer and the steric stabilizer polymer, buta non-solvent for the polymer shell being formed. For example, in analiphatic hydrocarbon solvent of Isopar G®, monomer methylmethacrylateis soluble; but after polymerization, the resultingpolymethylmethacrylate is not soluble.

The polymer shell formed from the monomer must be completelyincompatible or relatively incompatible with the solvent in which thecomposite pigment particles are dispersed. Suitable monomers may bethose described above, such as styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylateor the like.

The steric stabilizer polymer may be a reactive and polymerizablemacromonomer which adsorbs, becomes incorporated or is chemically bondedonto the surface of the polymer shell being formed. The macromonomer asa steric stabilizer, determines the particle size and colloidalstability of the system.

The macromonomer may be an acrylate-terminated or vinyl-terminatedmacromolecule, which are suitable because the acrylate or vinyl groupcan co-polymerize with the monomer in the reaction medium.

The macromonomer preferably has a long tail, R, which may stabilize thecomposite pigment particles in a hydrocarbon solvent.

One type of macromonomers is acrylate terminated polysiloxane (Gelest,MCR-M11, MCR-M17, MCR-M22), as shown below:

Another type of macromonomers which is suitable for the process isPE-PEO macromonomers, as shown below:

R_(m)O—[—CH₂CH₂O—]_(n)—CH₂—phenyl—CH═CH₂

or

R_(m)O—[—CH₂CH₂O—]_(n)—C(═O)—C(CH₃)═CH₂

The substituent R may be a polyethylene chain, n is 1-60 and m is 1-500.The synthesis of these compounds may be found in Dongri Chao et al.,Polymer Journal, Vol. 23, no.9, 1045 (1991) and Koichi Ito et al,Macromolecules, 1991, 24, 2348.

A further type of suitable macromonomers is PE macromonomers, as shownbelow:

CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The n, in this case, is 30-100. The synthesis of this type ofmacromonomers may be found in Seigou Kawaguchi et al, Designed Monomersand Polymers, 2000, 3, 263.

To incorporate functional groups for charge generation, a co-monomer maybe added in the reaction medium. The co-monomer may either directlycharge the composite pigment particles or have interaction with a chargecontrol agent in the display fluid to bring a desired charge polarityand charge density to the composite pigment particles. Suitableco-monomers may includevinylbenzylaminoethylamino-propyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid, 2-acrylamino-2-methylpropane sulfonic acid,2-(dimethylamino)ethyl methacrylate,N-[3-(dimethylamino)propyl]methacrylamide and the like.

Alternatively, the composite pigment particles may be prepared by livingradical dispersion polymerization, as shown in FIG. 2.

The living radical dispersion polymerization technique is similar to thedispersion polymerization described above by starting the process withpigment particles (21) and monomer dispersed in a reaction medium.

The monomers used in the process to form the shell (22) may includestyrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butylmethacrylate, t-butyl acrylate, t-butyl methacrylate, vinyl pyridine,n-vinyl pyrrolidone, 2-hydoxyethyl acrylate, 2-hydroxyethylmethacrylate, dimethylaminoethyl methacrylate and the like.

However in this alternative process, multiple living ends (24) areformed on the surface of the shell (22). The living ends may be createdby adding an agent such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy),a RAFT (reversible addition-fragmentation chain transfer) reagent or thelike, in the reaction medium, for the living radical polymerization.

In a further step, a second monomer is added to the reaction medium tocause the living ends (24) to react with the second monomer to form thesteric stabilizers (23). The second monomer may be lauryl acrylate,lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate,hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octylmethacrylate, n-octadecyl acrylate, n-octadecyl methacrylate or thelike.

The steric stabilizers should be compatible with the solvent in whichthe composite pigment particles are dispersed to facilitate dispersionof the composite pigment particles in the solvent.

The steric stabilizers may also be prepared through living radicalpolymerization.

A co-monomer may also be added to generate charge. Suitable co-monomersmay include vinylbenzylaminoethylaminopropyl-trimethoxysilane,methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid,vinyl phosphoric acid and the like.

Further alternatively, the composite pigment particles may be formed bycoating core pigment particles with polyurethane and/or polyurea.

Polyurethane and polyurea usually are not compatible to a non-polarhydrocarbon solvent and their hardness and elastic property can be tunedthrough the monomer composition.

In the composite pigment particles of the present invention, the shellmay be a polyurethane or polyurea material. The steric stabilizers maybe non-polar long chain hydrocarbon molecules.

The synthesis method is similar to emulsion or dispersionpolymerization, except that polycondensation occurs, inside micelles,with polyurethane monomer and the inorganic core pigment particles.

The polyurethane or polyurea coating system may be considered as anoil-in-oil emulsion, which contains two incompatible solvents, one ofwhich is a non-polar organic solvent and the other is a polar organicsolvent. The system may also be referred to as non-aqueous emulsionpolycondensation, in which the non-polar solvent is the continuous phaseand the polar solvent is the non-continuous phase. The monomer and theinorganic pigment particles are in the non-continuous phase. Suitablenon-polar solvents may include the solvents in the Isopar® series,cyclohexane, tetradecane, hexane or the like. The polar solvents mayinclude acetonitrile, DMF and the like.

An emulsifier or dispersant is critical for this biphasic organicsystem. The molecular structure of the emulsifier or dispersant maycontain one part soluble in the non-polar solvent, and another partanchoring to the polar phase. This will stabilize the micelles/dropletscontaining the monomer and the inorganic pigment particles and servingas a micro-reactor for the particle formation through polycondensation.

Suitable emulsifiers or dispersants may include di-block co-polymers,such as poly (isoprene)-b-poly(methyl methacrylate),polystyrene-b-poly(ethene-alt-propene) (Kraton) or the like.

Also, a co-emulsifier may be added to form chemical bonding with theparticles. For example, amine terminated hydrocarbon molecules can reactwith the particles during polycondensation and bond to surface as robuststeric stabilizers. Suitable co-emulsifiers may include surfonamine(B-60, B-100 or B-200) as shown below:

CH₃—[—OCH₂CH₂—]_(x)—[—OCH₂CH(CH₃)—]_(y)—NH₂

wherein x is 5-40 and y is 1-40.

An alternative approach is to continue growing polyacrylate stericstabilizers after the polycondensation reaction in the microreactor iscompleted. In this case, the shell is formed from polyurethane while thesteric stabilizers may be polyacrylate chains. After the emulsifier ordispersant used in the process is washed away from the particle surface,the composite pigment particles are stable in the non-polar solvent(i.e., display fluid) with the polyacrylate stabilizers. Some materialsthat can initiate acrylate polymerization include isocyanatoethylacrylate, isocyanatostyrene or the like.

Monomers for the steric stabilizer may be a mixture of hydroxyethylmethacrylate and other acrylate that are compatible to the non-polarsolvent, such as lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate,n-octadecyl methacrylate or the like.

In any of the processes described above, the quantities of the reagentsused (e.g., the inorganic core pigment particles, the shell material andthe material for forming the steric stabilizers) may be adjusted andcontrolled to achieve the desired organic content in the resultingcomposite pigment particles.

The third aspect of the present invention is directed to a display fluidcomprising the composite pigment particles of the present invention,which composite pigment particles are dispersed in a solvent. Apreferred solvent has a low dielectric constant (preferably about 2 to3), a high volume resistivity (preferably about 1015 ohm-cm or higher)and a low water solubility (preferably less than 10 parts per million).Suitable hydrocarbon solvents may include, but are not limited to,dodecane, tetradecane, the aliphatic hydrocarbons in the Isopar® series(Exxon, Houston, Tex.) and the like. The solvent can also be a mixtureof a hydrocarbon and a halogenated carbon or silicone oil base material.

The present invention is applicable to a one-particle, two-particle ormultiple particle electrophoretic display fluid system. In a multipleparticle system, there may be more than two types of pigment particlesand each type has a color which is different from the colors of othertypes.

In other words, the present invention may be directed to a display fluidcomprising only the composite pigment particles prepared according tothe present invention which are dispersed in a hydrocarbon solvent. Thecomposite pigment particles and the solvent have contrasting colors.

Alternatively, the present invention may be directed to a display fluidcomprising two types of pigment particles dispersed in an organicsolvent and at least one of the two types of the pigment particles isprepared according to the present invention. The two types of pigmentparticles carry opposite charge polarities and have contrasting colors.For example, the two types of pigment particles may be black and whiterespectively. In this case, the black particles may be preparedaccording to the present invention, or the white particles may beprepared according to the present invention, or both black and whiteparticles may be prepared according to the present invention.

The composite pigment particles prepared according to the presentinvention, when dispersed in an organic solvent, have many advantages.For example, the density of the composite pigment particles may besubstantially matched to the organic solvent, thus improving performanceof the display device. In other words, the difference between thedensity of the composite pigment particles and the density of thesolvent is less than 2 g/cm³, more preferably less than 1.5 g/cm³ andmost preferably less than 1 g/cm³.

In the two particle system, if only one type of the pigment particles isprepared according to the present invention, the other type of pigmentparticles may be prepared by any other methods. For example, theparticles may be polymer encapsulated pigment particles.Microencapsulation of the pigment particles may be accomplishedchemically or physically. Typical microencapsulation processes includeinterfacial polymerization/crosslinking, in-situpolymerization/crosslinking, phase separation, simple or complexcoacervation, electrostatic coating, spray drying, fluidized bed coatingand solvent evaporation, etc.

The composite pigment particles prepared by the previously knowntechniques may also exhibit a natural charge, or may be chargedexplicitly using a charge control agent, or may acquire a charge whensuspended in the organic solvent. Suitable charge control agents arewell known in the art; they may be polymeric or non-polymeric in nature,and may also be ionic or non-ionic, including ionic surfactants such assodium dodecylbenzenesulfonate, metal soap, polybutene succinimide,maleic anhydride copolymers, vinylpyridine copolymers, vinylpyrrolidonecopolymer, (meth)acrylic acid copolymers or N,N-dimethylaminoethyl(meth)acrylate copolymers), Alcolec LV30 (soy lecithin), Petrostep B100(petroleum sulfonate) or B70 (barium sulfonate), Solsperse 17000 (activepolymeric dispersant), Solsperse 9000 (active polymeric dispersant),OLOA 11000 (succinimide ashless dispersant), OLOA 1200 (polyisobutylenesuccinimides), Unithox 750 (ethoxylates), Petronate L (sodiumsulfonate), Disper BYK 101, 2095, 185, 116, 9077 & 220 and ANTI-TERRAseries.

EXAMPLES Example 1 Step A: Deposition ofVinylbenzylaminoethylaminopropyl-trimethoxysilane on Black PigmentParticles

To a 1 L reactor, Black 444 (Shepherd, 40 g), isopropanol (320 g), DIwater (12 g), ammonium hydroxide (28%, 0.4 g) and Z-6032 (Dow Corning,16 g, 40% in methanol) were added. The reactor was heated to 65° C. withmechanical stirring in a sonication bath. After 5 hours, the mixture wascentrifuged at 6000 rpm for 10 minutes. The solids were redispersed inisopropanol (300 g), centrifuged and dried at 50° C. under vacuumovernight to produce 38 g of desired pigment particles.

Step B: Preparation of Polymer Coating on Pigment Particles throughDispersion Polymerization

Two (2) g of polyvinylpyrrolidone (PVP K30) was dissolved in a mixtureof 94.5 g water and 5.5 g ethanol. The solution was purged with nitrogenfor 20 minutes and heated to 65° C. The pigment particles (4 g) preparedfrom Step A was dispersed in a mixture of 3.0 g lauryl acrylate, 0.2 gdivinylbezene and 0.03 g AIBN (azobisisobutyronitrile) to form a uniformsuspension. This suspension was added into the PVP solution at 65° C.With stirring, the polymerization reaction lasted about 12 hours.

Then a mixture of 3.0 g octadecyl acrylate and 0.03 g AIBN was addedinto the above reaction flask and the reaction was continued for 12hours.

The solids produced were separated from the liquid throughcentrifugation and then washed with isopropanol and methylethylketone toremove PVP K30 and other chemicals that were not bonded on the pigmentparticles. The solids were dried at 50° C. under vacuum to produce finalcomposite black particles. The organic content of the particles producedwas about 34% by weight, tested through TGA (thermal gravimetricanalysis).

Example 2 Synthesis of Colored Composite Pigment Particles

Hostaperm Red D3G 70-EDS (Clariant, 2.5 g), methyl methacrylate (8 g)and toluene (2 g) were added into a 20 ml vial and sonicated for 2hours. To a 250 mL reactor, the above mixture, MCR-M22 (Gelest, 5.7 g)and DMS-T01 (Gelest, 30 g) were added. The reactor was heated to 70° C.with magnetic stirring and purged with nitrogen for 20 minutes, followedby the addition of lauroyl peroxide (0.07 g). After 19 hours, themixture was centrifuged at 5000 rpm for 15 minutes. The solids producedwere redispersed in hexane and centrifuged. This cycle was repeatedtwice and the solids were dried at room temperature under vacuum toproduce the final particles. The polymer content of the particlesproduced was about 49% by weight, tested through TGA (thermalgravimetric analysis).

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention.

What is claimed is:
 1. An electrophoretic display fluid comprising acharge control agent and charged composite pigment particles dispersedin a hydrocarbon solvent, wherein each of said charged composite pigmentparticles consist of at least one core pigment particle, an organicpolymer shell, and steric stabilizer polymers, wherein each of thesteric stabilizer polymers is chemically bonded on the surface of theorganic polymer shell and each of the steric stabilizer polymers isselected from the group consisting of polyethylene, polypropylene,polyester, polysiloxane, and a mixture thereof.
 2. The electrophoreticdisplay fluid of claim 1, wherein the density of the charged compositepigment particles substantially matches the density of the hydrocarbonsolvent.
 3. The electrophoretic display fluid of claim 1, wherein thedifference between the density of the composite pigment particles andthe density of the hydrocarbon solvent is less than 2 g/cm3.
 4. Theelectrophoretic display fluid of claim 1, wherein said core pigmentparticle is an inorganic pigment particle.
 5. The electrophoreticdisplay fluid of claim 1, wherein said core pigment particle is anorganic pigment particle.
 6. The electrophoretic display fluid of claim1, wherein said core pigment particle is surface treated.
 7. Theelectrophoretic display fluid of claim 1, wherein said core pigmentparticle is not surface treated.
 8. The electrophoretic display fluid ofclaim 1, wherein said organic polymer shell is formed from polyacrylate,polyurethane, polyurea, polyethylene, polyester, or polysiloxane.
 9. Theelectrophoretic display fluid of claim 1, wherein said organic polymershell is completely incompatible or relatively incompatible with thehydrocarbon solvent.
 10. The electrophoretic display fluid of claim 1,wherein the surface of the organic polymer shell comprises functionalgroups to enable charge generation or interaction with the chargecontrol agent.
 11. The electrophoretic display fluid of claim 1, whereinsaid organic polymer shell is formed from a polymer material having apolar functionality on a main chain or a side chain.
 12. Theelectrophoretic display fluid of claim 11, wherein the polarfunctionality is —COOH, —OH, —NH2, R—O—R or R—NH—R, wherein R is analkyl or aryl group.
 13. The electrophoretic display fluid of claim 11,wherein the side chain has an aromatic moiety.
 14. The electrophoreticdisplay fluid of claim 1, wherein the composite pigment particles aremade from dispersion polymerization or living radical dispersionpolymerization.
 15. The electrophoretic display fluid of claim 1,further comprising a second type of charged composite pigment particles.16. The electrophoretic display fluid of claim 15, wherein each of saidsecond type of charged composite pigment particles consist of at least acore pigment particle, an organic polymer shell, and steric stabilizerpolymers, wherein each of the steric stabilizer polymers is chemicallybonded on the surface of the organic polymer shell; and the two types ofcharged pigment particles are of contrasting colors.
 17. Theelectrophoretic display fluid of claim 15, wherein the second type ofcharged composite pigment particles are formed from a conventionalmicroencapsulation technique.
 18. The electrophoretic display fluid ofclaim 1, wherein said organic polymer shell is formed from monomersselected from the group consisting of styrene, methyl acrylate, methylmethacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate,t-butyl methacrylate, vinyl pyridine, n-vinyl pyrrolidone, 2-hydoxyethylacrylate, 2-hydroxyethyl methacrylate, and dimethylaminoethylmethacrylate.